powerpc/mm: Fix some SMP issues with MMU context handling

This patch fixes a couple of issues that can happen as a result
of steal_context() dropping the context_lock when all possible
PIDs are ineligible for stealing (hopefully an extremely hard to
hit occurence).

This case exposes the possibility of a stale context_mm[] entry
to be seen since destroy_context() doesn't clear it and the free
map isn't re-tested. It also means steal_context() will not notice
a context freed while the lock was help, thus possibly trying to
steal a context when a free one was available.

This fixes it by always returning to the caller from steal_context
when it dropped the lock with a return value that causes the
caller to re-samble the number of free contexts, along with
properly clearing the context_mm[] array for destroyed contexts.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>